I understand it like this. * empty - Are there no more values? * front - Get me the current value. * popFront - Advance to the next value.
In terms of how I implement an InputRange in general, I typically end up with this.
* empty - Advance and cache "current value," return true if we ran out of values. * front - enforce(!empty), which in turn caches the current value, which we then return. * popFront - enforce(!empty), clear the cached value so we can later advance.
So .front gives you the same thing again and again until you call popFront, you could indeed call .front before .empty, but you may get an exception. This obviously isn't how I implement all InputRanges, as there are better ways to write other ranges, such as ranges which operate on sets of integers or wrap arrays. This is just my last resort general case InputRange implementation.
.front assignment obviously replaces the cached value.
